1. Field of the Invention
The present invention relates to a photosensitive member of the function-separated type comprising an amorphous silicon:germanium layer as a charge generating layer and a hydrogen-containing amorphous carbon layer as a charge transporting layer.
2. Description of the Prior Art
Remarkable progress has been made in the application of electrophotographic techniques since the invention of the Carlson process. Various materials have also been developed for use in electrophotographic photosensitive members.
Conventional photoconductive materials chiefly include inorganic compounds such as amorphous selenium, selenium-arsenic, selenium-tellurium, zinc oxide, amorphous silicon and the like, and organic compounds such as polyvinylcarbazole, metal phthalocyanine, dis-azo pigments, tris-azo pigments, perillene pigments, triphenymethanes, triphenylamines, hydrazones, styryl compounds, pyrazolines, oxazoles oxadiazoles and the like. The structures of photosensitive members include, for example, those of the single-layer type wherein such a material is used singly, the binder type wherein the material is dispersed in a binder, and the function-separated type comprising a charge generating layer and a charge transporting layer.
However, conventional photoconductive materials have various drawbacks. For example, the above-mentioned inorganic materials, except for amorphous silicon (a-Si), are harmful to the human body.
The electrophotographic photosensitive member, when employed in a copying apparatus, must always have stabilized characteristics even if it is subjected to the severe environmental conditions of charging, exposure, developing, image transfer, removal of residual charges and cleaning. However, the above described organic compounds have poor durability and many unstable properties.
In order to eliminate these drawbacks, progress has been made in recent years in the application of a-Si formed by the glow discharge process to an electrophotographic photosensitive members as a material with reduced harmfulness, higher sensitivity, higher hardness (such as more than 7H level of the JIS standards for pencil lead hardness) and higher durability. Nevertheless, a-Si is hazardous to manufacture since it requires highly ignitable silane gas as its starting material. Moreover, a-Si requires a large quantity of silane gas which is expensive, rendering the resulting photosensitive member exceedingly more costly than conventional photosensitive members. The manufacture of photosensitive members of a-Si involves many disadvantages. For example, a-Si is low in film-forming speed and releases a large amount of explosive undecomposed silane products in the form of particles when forming a film. Such particles, when incorporated into the photosensitive member being produced, adversely influences the quality of images obtained. Further, a-Si has a low chargeability due to its original high relative dielectric constant. This necessitates the use of a charger of higher output for charging the a-Si photosensitive member to a predetermined surface potential in the copying apparatus.
On the other hand, it has been proposed in recent years to use amorphous carbon films as plasma-polymerized organic films for photosensitive members.
Plasma-polymerized organic films per se have been well-known for a long time. In Journal of Applied Polymer Science, Vol. 17, pp. 885-892, 1973, for example, M. Shen and A. T. Bell state that a plasma-polymerized organic film can be produced from the gas of any organic compound. The same authors discuss film formation by plasma polymerization in "Plasma Polymerization," published by the American Chemical Society in 1979.
However, the plasma-polymerized organic films prepared by the conventional process have been used only as insulating films. They are thought to be insulating films having a specific resistivity of about 10.sup.16 ohm-cm like usual polyethylene films, or are used, as recognized, at least as such. The use of the film in electrophotographic photosensitive members is based on the same concept; the film has found limited use only as an undercoat or overcoat serving solely as a protective layer, adhesion layer, blocking layer or insulating layer.
For example, Unexamined Japanese Patent Publication SHO 59-28161 discloses a photosensitive member which comprises a plasma-polymerized high polymer layer of reticular structure formed on a substrate and serving as a blocking-adhesion layer, and an a-Si layer formed on the polymer layer. Unexamined Japanese Patent Publication SHO 59-38753 discloses a photosensitive member which comprises a plasma-polymerized film having a thickness of 10 to 100 angstroms and formed over a substrate as a blocking-adhesion layer, and an a-Si layer formed on the film, the plasma-polymerized film being prepared from a gas mixture of oxygen, nitrogen and a hydrocarbon and having a high resistivity of 10.sup.13 to 10.sup.15 ohm-cm. Unexamined Japanese Patent Publication SHO 59-136742 discloses a photosensitive member wherein an aluminum substrate is directly coated with a carbon film having a thickness of about 1 to about 5 microns and serving as a protective layer for preventing aluminum atoms from diffusing through an a-Si layer formed over the substrate when the member is exposed to light. Unexamined Japanese Patent Publication SHO 60- 63541 discloses a photosensitive member wherein a diamond-like carbon film, 200 angstroms to 2 microns in thickness, is interposed between an aluminum substrate and an overlying a-Si layer to serve as an adhesion layer to improve the adhesion between the substrate and the a-Si layer. The publication says that the film thickness is preferably up to 2 microns in view of the residual charge.
These disclosed inventions are all directed to a so-called undercoat provided between the substrate and the a-Si layer. In fact, these publications mention nothing whatever about charge transporting properties, nor do they offer any solution to the foregoing substantial problems of a-Si.
Furthermore, U.S. Pat. No. 3,956,525, for example, discloses a photosensitive member of the polyvinylcarbazole-selenium type coated with polymer film having a thickness of 0.1 to 1 microns and formed by glow discharge polymerization as a protective layer. Unexamined Japanese Patent Publication SHO 59-214859 discloses a technique for protecting the surface of an a-Si photosensitive member with an approximately 5-micron-thick film formed by plasma-polymerizing an organic hydrocarbon monomer such as styrene or acetylene. Unexamined Japanese Patent Publication SHO 60-61761 discloses a photosensitive member having a diamond-like carbon thin film 500 angstroms to 2 microns thick and serving as a surface protective layer, it being preferred that the film thickness be up to 2 microns in view of trasmittancy. Unexamined Japanese Patent Publication SHO 60-249115 discloses a technique for forming a film of amorphous carbon or hard carbon with a thickness of about 0.05 to about 5 microns for use as a surface protective layer. The publications states that the film adversely affects the activity of the protected photosensitive member it exceeds 5 microns in thickness.
These disclosed inventions are all directed to a so-called overcoat formed over the surface of the photosensitive member. The publications disclose nothing whatever about charge transporting properties, nor do they solve the aforementioned substantial problems of a-Si in any way.
Unexamined Japanese Patent Publication SHO 50-46130 discloses an electrophotographic photosensitive member of the polyvinylcarbazole type which has a polymer film 0.001 to 3 microns thick, which is formed on its surface by being subjected to glow discharge polymerization. Nevertheless, the publication is totally mute about charge transporting properties, further failing to solve the foregoing substantial problems of a-Si.
With respect to a-Si, W. E. Spear and P. G. LeComber state in Philosophical Magazine, Vol. 33, pp. 935-949, 1976, that a-Si can control polarities. Since then, progress has been made in the application of a-Si to various photosensitive devices.
For example, Unexamined Japanese Patent Publication No. SHO 56-62254 discloses a photosensitive member of a-Si containing carbon. This reference aims at adjusting the photoconductivity of a-Si by incorporating carbon therein. The a-Si layer needed has a large thickness.
Thus, the conventional plasma-polymerized organic films for use in electrophotographic photosensitive members are used as undercoats or overcoats because of their insulating properties and need not have a carrier transporting function. Accordingly, the films used have limited the thicknesses to a very small value up to about 5 microns at the largest. Carriers pass through the film owing to a tunnel effect, while if the tunnel effect is not expectable, the film used has such a small thickness that will not pose problems due to the occurrence of a residual potential. Further, the conventional a-Si layer, for use in electrophotographic photosensitive members, are used with a large thickness, causing disadvantages in view of cost or productivity.